The range of wavelengths propagated by optical fibers and waveguides is such that this medium is well suited to broadband, long haul communications. Optical fibers, for example, have a low loss window in the 1.3 to 1.55 .mu.m range such that properly designed optical transmission systems can transfer optical signals over long distances without intermediate repeaters. Although optical fibers can transmit bandwidths at or approaching the Tera bit range, limitations such as electronic circuitry prevent full utilization of this characteristic. Thus, to more fully exploit this bandwidth capability, it is known to divide the optimum wavelength range into wavelength channels and to impress multiple data streams onto those channels.
By this process, known as wavelength division multiplexing (WDM), parallel data chains carried on separated wavelength channels are launched into an optical fiber and subsequently demultiplexed at the receiver end into the same wavelength channels.
It is also possible to employ multiple wavelengths in switching. For example, tunable laser transmitters can be tuned to different wavelengths and data transmission from each transmitter is mixed together optically on a fiber. At the receiver end the transmission is separated again using fixed wavelength discriminating optical filters and conventional photodetectors. In this system switching is achieved simply by tuning each transmitter to the wavelength of the intended receiver, rather like a broadcast radio system but in reverse. In other such systems of this kind both the transmitter and receiver or the receiver alone can be tuned over a range of wavelengths. Optionally, optical devices, capable of changing the wavelength of light, can be used.
Another example of the use of wavelength division in switching is analogous to the Code Division Multiplex System (CDMA or "spread spectrum") technique used in certain mobile radio telephone systems. In the present optical case, each channel transmitted into the optical bus contains a number of wavelengths in a pattern that is different from the pattern of every other transmitter. Reception of a selected channel is achieved by correlation of the expected pattern (spectrum shape) with the received signal which contains both the desired pattern as well as all other transmissions. By a proper choice of the spectral pattern, one can achieve separation of the desired signal because all other signals appear as noise. In effect the noise has a low correlation coefficient with the expected pattern while the desired signal exhibits a high correlation.
In order to maximize the information transfer within the aforementioned low loss window, as many wavelength channels as possible are utilized. This, of course, requires narrow optical bandwidth of each channel and good separation between channels.
The development of such transmission systems has been hampered by the lack of optical components which economically generate, detect, separate and convert optical wavelengths in a narrow range.
Heretofore, optical systems have relied on free space optics with gratings and lenses followed by semiconductor detectors. Spatial wavelength separation has been accomplished by integrated waveguide optics relying on the diffractive effect of multi-path interference. Other prior art optical components have relied on light coupling through index matching of parallel waveguides as well as programmable holograms and acousto-optical tunable filters.